1. Field of the Invention
The present invention relates generally to use of photometric exposure and measurement to monitor and control quality of radiographic images. More specifically, the present invention relates to a method and apparatus for producing and sensing at least two optical densities generated on photo sensitive material by exposing said material to two sources of even illumination at a correct frequency. The two intensities differ by a precise, known amount. The apparatus determines density, gamma, base plus fog and the process quality factor of the film.
2. Background and Prior Art
It has long been known in the art of film processing to precisely expose a piece of film, process the film, and then measure the optical density of the exposed area.
Instruments for exposing photographic material to the series of graduated light intensities or for precisely known periods are called "sensitometers". They generally consist of a light source of known intensity and a means for producing exposures that differ by known amounts.
Several types of sensitometers are well known to the prior art. Exposure may be varied by controlling either light intensity or exposure time. One method used is a rotating sector wheel that allows light to pass for a smaller period of time as distance from its axis of rotation increases; another uses areas of differing density between a precise amplitude light source and photosensitive material, which produces a step-wedge. Many variations exist of these instruments, but their general operation is similar, variations being only for application to a specific job.
Once the exposed film is developed, a device called a "densitometer" is utilized to measure the optical density of the exposed area. To properly understand the present invention it must be understood at the onset that all precision densitometers taught by the prior art require the use of complex optical systems. These complex optical systems are required to precisely control the amount and type of light emerging from the sensed film. See, for example, FIG. A2 on page 14 of ANSI PH 2.19-1976, "Conditions for diffuse and doubly diffuse transmission measurements" (copy attached).
Density is properly defined by the equation: EQU density=log (I.sub.o /I.sub.t)
where I.sub.o is the light incident on the sample and I.sub.t is the light transmitted. Thus the measure of density is fundamentally a comparison of two light intensities. However, the silver deposits which form the image on a photographic or radiographic film scatters light as well as absorbs it. Therefore, the actual numerical value of the density of a particular area of film will depend on the method by which it is measured.
If a totally parallel beam of light falls on a processed film, only part of the light is absorbed by the silver in the film. A second part passes through in the same direction as the transmitted beam and a third part is scattered. Use of specular light input and diffuse light output by conventional densitometers requires the use of a complex optical system that is very sensitive to optical geometry, i.e. changes in the sensed film position in the optical path. This makes measurement of density difficult to accomplish with any substantial degree of precision as a repeatable value from instrument to instrument.
In discussing this problem, the Radiographic Markets Division of Eastman Kodak states, at page 29 of "Sensiometric Properties of X-Ray Films" (copy attached),
"It is, therefore, not surprising that if the same film area is read on a number of different densitometers, the numerical results will differ somewhat, since densitometers differ somewhat in optical design. Variations from one type of commercial densitometer to another will in general be small, but the possibility must be kept in mind when planning precision work." (page 29) PA0 1. Variables associated with the nature of the subject and the radiographic conditions under which the radiograph was exposed; or PA0 2. Variables associated with the type of film/screens and the development process.
The complex optical systems of commercial densitometers cost thousands of dollars. It is technically infeasible, as well as economically impossible, to build this type of precision measuring equipment into apparatus for use for routine quality control in normal clinical radiography. Further, even if built, such conventional equipment would be so sensitive to film position that it would have to be operated by a skilled technician to obtain consistant results.
The purpose of obtaining accurate density measurements is to enable the operator to generate a curve, known as the "H&D" curve or the density-log exposure (D-log E curve); to measure the speed of the film, and to evaluate processor performance by comparing this information against a standard.
The D-log E curve comprises three parts: a toe, a straight line portion and a shoulder. The toe is a region of underexposure. Changes of exposure in this area have little effect on developed optical density. The curve's shoulder is a region of overexposure. Again, changes in exposure cause only small changes in density. The curve's straight line portion is the region of correct exposure. In this straight line portion, optical density of the developed film is a direct function of the logarithm of exposure.
Persons skilled in the art of sensitometry plot D-log E curves to obtain a value known as "gamma". Gamma may be represented geometrically by the tangent of the angle formed by a line tangent to the straight line portion of the D-log E curve and the curve's exposure axis. Gamma is a measure of film contrast. For any particular film type there will be a characteristic family of gamma curves that indicate different film speeds, or developed densities, for a given exposure. If exposure is held constant, each curve represents a change in processor variables, i.e. time, temperature and/or processor chemistry. This is another way of saying that film speed or sensitivity is inversely proportional to exposure time required to achieve a specific developed density. Thus, at a fixed relative exposure, any change in density will reflect a change in speed; which is also a function of processor variables.
It is therefore possible to calculate change from a reference standard in the value of gamma and speed, for a fixed exposure, solely as a function of film and processor variables. The optimal value of gamma, speed and base-plus-fog for a given film indicates the film was correctly processed and thus has the optimum combination of contrast, resolution and base-plus-fog for that particular film type. Conversely, a deviation in speed or gamma indicates the film was processed incorrectly and will produce an image of poor diagnostic quality.
As a result of this known relationship between the value of gamma, film speed and the quality of the film processor, gamma and developed density (speed change) are sensitive and useful indicators for determining the optimum operating point for film processing systems.
It will readily be appreciated that the prior art has a number of shortcomings. The precision densitometers required by the prior art to determine density are expensive and delicate. Such instruments are prone to break down and tend to become inaccurate after a period of time, thus necessitating expensive recalibration.
Another drawback of the prior art is the requirement that measurement of a precise optical density requires significant use of highly trained manpower. A trained technician first has to carefully obtain a number of precise density readings and then calculate the gamma of the film. As a result, gamma measurements tended to be taken seldom, usually only at the beginning of an operating day on a given processor, and are repeated only when it becomes grossly obvious to the radiologist that a processor is not performing well.
This last drawback has very serious implications for the public health. Every hospital of significant size in the United States has a number of fully automatic x-ray film processors. The film processed in these units is exposed by passing a dose of x-ray radiation through a patient. The film is then processed by a technician in an automatic film processor. If the processor does not work optimally, the result is a poor quality image on the x-ray film. Unfortunately a similar poor quality image is produced if the x-ray was improperly exposed due to a mistake in radiographic technique, i.e. a wrong setting of kilovoltage, or x-ray current, or time of exposure.
For the purpose of the present invention this means quality of medical radiographs generally is a function of either:
At the present time it is a normal procedure in most hospitals for the x-ray exposure to be repeated if a poor quality image is obtained. This exposes the patient to additional radiation, which is undesirable from a public health standpoint. Often the fact that the poor quality radiograph is due to processor errors rather than faulty x-ray exposure only becomes apparent after a patient has been x-rayed several times. It is also unfortunate that it is common practice in most hospitals for the radiology technician to increase the radiographic technique (increase the x-ray dose to the patient) to gain better film contrast in order to compensate for a poorly functioning processor. This practice has become common because there is never enough time or trained personnel available to use the quality control techniques taught by the prior art.
No prior art known to the inventor teaches precisely evaluating gamma and speed with sufficient accuracy in a short enough time to allow a routine measurement of processor quality, quality of film batches, or proper x-ray exposure to be made prior to re-exposing the patient.
It is therefore an object of the present invention to provide a means of measuring the density, speed change, gamma and processor quality of a radiographic film without requiring the use of precise, geometrically dependent optics.
It is a further object of the present invention to provide a means of measuring processor quality, evaluating film batch quality and checking on x-ray patient exposure that is sufficiently rapid and automatic that such measurements may be made on a routine basis by an unskilled person.
It is yet another purpose of the present invention to provide a means of measuring processor quality that is inexpensive and rugged.
It is yet a further object of the present invention to provide a quick and simple method of determining the processor quality and other sensitometric variables of every film exposed by a radiographic system; or, alternatively, to provide a quick and simple method for running test films at periodic intervals.
The closest prior art to the present invention known to the inventor is the system described in Medical Radiography and Photography, Vol. 49, No. 1, pp. 2-6, 28, 1973, entitled "A Simple Method of Processor Control", by Daniel J. Lawrence (copy attached).